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  • Plaxis finite element code for soil and rock analyses

    issue 24 / October 2008 Plaxis Bulletin

    Capacity Analysis of Suction Anchors in Clay by Plaxis 3D Foundation On Stability Analysis of Slurry-Wall Trenches

    Seabed instability and 3D FE jack-up soil-structure interaction analysis

  • 2

    Colophon

    the Plaxis Bulletin is the combined magazine of Plaxis B.V. and the Plaxis Users

    association (nl). the Bulletin focuses on the use of the finite element method in geotech-

    nical engineering practise and includes articles on the practical application of the Plaxis

    programs, case studies and backgrounds on the models implemented in Plaxis.

    the Bulletin offers a platform where users of Plaxis can share ideas and experiences with

    each other. the editors welcome submission of papers for the Plaxis Bulletin that fall in

    any of these categories.

    the manuscript should preferably be submitted in an electronic format, formatted as

    plain text without formatting. it should include the title of the paper, the name(s) of the

    authors and contact information (preferably email) for the corresponding author(s). the

    main body of the article should be divided into appropriate sections and, if necessary,

    subsections. if any references are used, they should be listed at the end of the article.

    the author should ensure that the article is written clearly for ease of reading.

    in case figures are used in the text, it should be indicated where they should be placed

    approximately in the text. the figures themselves have to be supplied separately from the

    text in a common graphics format (e.g. tif, gif, png, jpg, wmf, cdr or eps formats are all

    acceptable). if bitmaps or scanned figures are used the author should ensure that they

    have a resolution of at least 300 dpi at the size they will be printed. the use of colour in

    figures is encouraged, as the Plaxis Bulletin is printed in full-colour.

    any correspondence regarding the Plaxis Bulletin can be sent by email to

    bulletin@plaxis.nl

    or by regular mail to:

    Plaxis Bulletin c/o erwin Beernink

    Po Box 572

    2600 an delft

    the netherlands

    the Plaxis Bulletin has a total circulation of 15.000 copies and is distributed worldwide.

    Editorial Board:

    Wout Broere Ronald Brinkgreve Erwin Beernink arny lengkeek

    Editorial

    New Developments

    Plaxis Practice Capacity Analysis of

    Suction Anchors in Clay by Plaxis 3D Foundation

    Plaxis Practice On Stability Analysis

    of Slurry-Wall Trenches

    Plaxis Practice Seabed instability and 3D FE jack-up soil-structure

    interaction analysis

    Recent Activities

    Activities 2008 - 2009

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    Editorial

    regarding ‘Plaxis Practice’, it is interesting to see that all three articles involve 3d cal-

    culations. Hence, there is a clear trend to perform 3d calculations, at least for complex

    geo-engineering projects. at the same time, the use of 2d fem is still increasing, since

    Plaxis 2d is used more and more for daily geotechnical design.

    the first article describes an evaluation of suction anchor bearing capacity with the 3d

    foundation program. in general, 3d models are not as accurate as 2d models. as a re-

    sult, ultimate limit states (such as safety factors or bearing capacities) may be over-

    estimated. it is demonstrated that interface elements play a crucial role in the accurate

    prediction of the suction anchor bearing capacity.

    the second article involves the stability analysis of slurry wall trenches. the situation is

    during construction is clearly three-dimensional. in addition to safety factor analysis, the

    authors describe a probabilistic design method. the results seem to be in good agreement

    with the geo-engineering practice.

    in the third article a 3d analysis of a complex offshore foundation is described. instability

    of the seabed is an important issue here. Gravel banks were proposed to stabilize the

    foundation. in the calculation different loading situations were considered. a good and

    stable solution could be obtained for the designed foundation. the structure has been

    build successfully and behaved well according to the predictions.

    Hereby we trust to have compiled again an interesting Plaxis bulletin for you. do not hesi-

    tate to contact us with your response on one of the published articles, or with new articles

    for future Bulletins. We are looking forward to receive many contributions.

    the editors

    a new Plaxis Bulletin is in front of you, with interesting articles about Plaxis applica-

    tions, new developments and a full agenda of activities. We are also pleased to an-

    nounce that new Plaxis products have been released, which are the 2D version 9.0,

    3D Foundation 2.2 and Plaxis-GiD. The latter is a general CaD-like 3D pre-processor

    which has been configured to address the Plaxis calculation kernel.

    Ronald Brinkgreve

  • 4

    New Developments

    in the previous Bulletin information was given about the Plaxis 3d developments. it is a

    pleasure to mention now that the fi rst general 3d program (Plaxis-Gid) has been released.

    this program is available as a service to those who feel restricted by the geometrical

    limitations of 3d tunnel or 3d foundation. more information can be obtained from the

    Plaxis sales department.

    in this Bulletin i like to mention another new development that is currently in progress:

    fully coupled fl ow-deformation analysis.

    most Plaxis users are familiar with the consolidation option in Plaxis 2d and 3d. so far,

    Plaxis has only considered Biot coupled consolidation under saturated conditions, forming

    a coupling between deformation and excess pore pressures. this works well for cases with

    constant hydraulic conditions, where the time interval or loading rate is such that the

    situation is neither fully drained nor fully undrained. for cases with changing hydraulic

    conditions, a simplifi ed solution is available by combining the standard Plaxis program

    with the transient fl ow module Plaxflow. However, if pore pressure is infl uenced by loading

    of (partially) undrained soil as well as changing hydraulic conditions, there is a need for

    consolidation based on total pore pressures, i.e. fully coupled fl ow-deformation analysis.

    examples where this type of analyses is required are clay embankments in tidal areas or

    excavations with dewatering in medium soft soils.

    Ronald Brinkgreve

    With the change of consolidation based on excess pore pressure to total pore pressure it

    becomes important to consider the phreatic surface and the unsaturated zone above. as a

    result of loading or changing hydraulic conditions, ground water fl ow may occur, and the

    position of the phreatic surface may change. soil that has been fully saturated may be-

    come unsaturated or vice versa. Hence, together with the implementation of fully coupled

    fl ow-deformation analysis, there is also need for models that can describe unsaturated

    soil behaviour in more detail. first of all, there are the Van Genuchten relationships be-

    tween suction, relative permeability and degree of saturation, which are also used in

    Plaxflow. secondly, there is the well-known Barcelona Basic model that deals with suction

    and swelling in the unsaturated zone. all this is implemented in the Plaxis calculation

    kernel to complete the fully coupled fl ow-deformation analysis feature.

    these new features will be available in Plaxis 2d version 9.1, which is planned for release

    mid 2009. When the implementation is ready, we can start beta-testing with a selected

    group of users. after implementation in Plaxis 2d we will proceed with the implementation

    in the 3d calculation kernel. We are confi dent that the new features will help many users

    in analysing their coupled and unsaturated soil problems.

    ronald Brinkgreve

    Plaxis bv

    New Developments

  • 5

    Plaxis Practice

    lars andresen, PhD, NGi, Oslo, Norway lewis Edgers, PhD, PE, Tufts University, Medford, Ma Usa Hans Petter Jostad, PhD, NGi, Oslo, Norway

    Capacity Analysis of Suction Anchors in Clay by Plaxis 3D Foundation

    Introduction this article describes the use of Plaxis 3d foundation v. 2.1 (Plaxis, 2008) to compute

    the undrained capacity of a suction anchor in clay. the objective of this study was to

    evaluate the performance of Plaxis 3d foundation for analyzing this particular problem

    by comparing the Plaxis 3d foundation results with results from other software including

    Plaxis 2d and nGi in-house codes. the effects of mesh fineness, use of interface elements

    and the wall roughness on the calculated capacity were also studied. there are several

    other aspects in the design of skirted anchors in clay which is not covered in this article.

    the reader is referred to andersen and Jostad (1999). a particular issue that this study

    focused on was use of interface elements adjacent to cylindrical suction anchors. the lack

    of isoparametric interface elements in the 2.1